In physical chemistry, mineralogy, and
materials science, a phase diagram is a type of graph used to show the equilibrium conditions
between the thermodynamically-distinct phases. In mathematics and physics, a phase diagram also has an alternative meaning,
as a synonym for a phase space.
Pressure-temperature diagrams
The simplest phase diagrams are pressure-temperature diagrams of a single simple substance, such as water. The axes correspond to the pressure and temperature. The phase diagram shows, in pressure-temperature
space, the lines of equilibrium or phase boundaries between the three phases of solid,
liquid, and gas.
A typical phase diagram. The dotted line gives the anomalous behaviour of water. The green lines mark the
freezing point and the blue line the
boiling point, showing how
they vary with pressure.
The markings on the phase diagram show the points where the free energy is non-analytic. The open spaces, where the
free energy is analytic, correspond
to the phases. The phases are separated by lines of non-analyticity, where phase
transitions occur, which are called phase boundaries.
In the diagram, the phase boundary between liquid and gas does not continue indefinitely. Instead, it terminates at a point on
the phase diagram called the critical point. This reflects the fact
that, at extremely high temperatures and pressures, the liquid and gaseous phases become indistinguishable, in what is known as a
supercritical fluid. In water, the critical point occurs at around 647
K (374 °C or 705 °F) and 22.064 MPa.
The existence of the liquid-gas critical point reveals a slight ambiguity in our above definitions. When going from the liquid
to the gaseous phase, one usually crosses the phase boundary, but it is possible to choose a path that never crosses the boundary
by going to the right of the critical point. Thus, the liquid and gaseous phases can blend continuously into each other. However,
it is impossible for the solid-liquid phase boundary to end in a critical point in the same way as the liquid-gas boundary,
because the solid and liquid phases have different symmetry[citation needed].
An interesting thing to note is that the solid-liquid phase boundary in the phase diagram of most substances, such as the one
shown above, has a positive slope. This is due to the solid phase having a higher density than the liquid, so that increasing the pressure increases the melting temperature, that is the
temperature at which metal melts. However, in the phase diagram for water the solid-liquid phase boundary has a negative slope.
This reflects the fact that ice has a lower density than water, which is an unusual property for a material.
Binary phase diagrams
Other much more complex types of phase diagrams can be constructed, particularly when more than one pure component is present.
In that case concentration becomes an important variable. Phase diagrams with more than
two dimensions can be constructed that show the effect of more than two variables on the phase of a substance. Phase diagrams can
use other variables in addition to or in place of temperature and pressure and composition, for example the strength of an
applied electrical or magnetic field and they can also involve substances that take on more than just three states of matter.
A phase diagram for a binary system displaying a eutectic point.
One type of phase diagram plots temperature against the relative concentrations of two substances in a binary mixture called a
binary phase diagram. Such a mixture can be either a solid solution,
eutectic or peritectic, among others. These two
types of mixtures result in very different graphs. A textbook example of a eutectic phase diagram is that of the olivine (forsterite and fayalite)
system.
Boiling Point Diagram
Another type of binary phase diagram is a boiling point diagram for a mixture of two components, i. e. chemical compounds. For two particular volatile components at a
certain pressure such as atmospheric pressure, a boiling point diagram shows what
vapor (gas) compositions are in equilibrium with
given liquid compositions depending on temperature. In a typical binary boiling point diagram, temperature is plotted on a
vertical axis and mixture composition on a horizontal axis. A simple example diagram with hypothetical components 1 and 2 in a
non-azeotropic mixture is shown at right. The fact that there are two separate curved lines
joining the boiling points of the pure components means that the vapor composition is usually not the same as the liquid
composition the vapor is in equilibrium with. See Vapor-Liquid Equilibrium for
a fuller discussion.
In addition to the above mentioned types of phase diagrams, there are thousands of other possible combinations. Some of the
major features of phase diagrams include congruent points, where a solid phase transforms directly into a liquid. There is also
the peritectoid, a point where two solid phases combine into one solid phase during heating. The inverse of this, when one solid
phase transforms into two solid phases during heating, is called the eutectoid.
A complex phase diagram of great technological importance is that of the iron-carbon system for less than 7% carbon (see steel).
The x-axis of such a diagram represents the concentration variable of the mixture. As
the mixtures are typically far from dilute and their density as a function of temperature usually unknown the preferred
concentration measure is mole fraction. A volume based measure like molarity would be unadvisable.
Common components of a phase diagram
Lines of equilibrium or phase boundaries refer to the lines that demarcate where phase transitions occur.
A triple point is, in a pressure-temperature phase diagram, the unique intersection of
the lines of equilibrium between three states of matter, usually solid,
liquid, and gas.
For a phase diagram with temperature on the vertical axis, a solidus is a line
below which the substance is stable in the solid state. A liquidus is a line above which the
substance is stable in a liquid state. There may be a gap between the solidus and liquidus; within the gap, the substance is
replaced by a mixture of solid crystals and liquid.
Liquid crystal phase diagrams
In liquid crystal physics, phase diagrams are used in the case of mixing of
nematogenic compounds to distinguish between the isotropic liquid phase, the nematic liquid phase.
See also
External links
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